Improving Immunogenicity of Subunit Vaccines Using Nanocarrier Systems

New generation vaccines, which are subunit vaccines, have considerably improved safety profiles, but are often poorly immunogenic, especially when administered through mucosal routes. Also, the diversity of the mechanisms of action of infectious pathogens requires the tuning of vaccine formulations towards humoral or cell mediated immunity, systemic or mucosal immunity, Th1 or Th2 immune responses. To modulate the immune response of subunit vaccines, the new field of immunobioengineering is developing a broad spectrum of nanocarrier systems. In this work, we describe the potential of two different fully synthetic nanocarrier systems to increase and modulate the immune responses of subunit vaccines. We show that upon intranasal administration, 50 nm solid-core NPs penetrated the nasal mucosae, transited via M cells, and were taken up by antigen- presenting cells in the nasopharynx-associated lymphoid tissue (NALT). Ovalbumin-conjugated NPs induced antigen-specific cytotoxic T lymphocytic responses in lung and spleen tissues, as well as humoral responses in mucosal airways. Co-conjugation of the TLR5 ligand flagellin to the NPs further enhanced the humoral responses in the airway as well as in the distant vaginal and rectal mucosal compartments, and induced cellular immune responses with a Th1 bias, in contrast to free flagellin. Furthermore, by increasing the size of the NPs from 30 to 200 nm, the conjugated protein antigen was more effectively delivered into both MHC class I and MHC class II-presentation pathways. Intranasal immunization with larger ovalbumin-conjugated NPs increased the magnitude of CD4 T cell responses in the lungs, as well as systemic and mucosal humoral responses. Most importantly, 200 nm NPs increased the proportion of antigen-specific polyfunctional CD4 T cells as compared to 30 nm NPs, resulting in a very interesting antigen nanocarrier for prophylactic vaccines against mucosal pathogens that require multifunctional CD4 T cells for protection, such Mycobacterium tuberculosis. In the last part of this thesis, the role of the nanocarrier polymersomes in the induction of adaptive immunity is characterized. Mice subcutaneously immunized with antigen-loaded into polymersomes and CpG displayed enhanced induction of antigen-specific CD4 T cells in spleen, lymph nodes and lungs, as compared to mice immunized not only with free antigen and CpG but also to the solid-core NP formulations. In contrast, unlike NPs, polymersomes did not enhance clonal expansion of antigen-specific CD8 T cells, nor cytokine production by effector CD8 T cells. Interestingly, a formulation with both polymersomes and NPs elicited T cell immunity characteristic of the two nanocarriers at the same time. In conclusion, this thesis proposes solid-core NPs and polymersomes as effective nanocarrier systems for the fine-tuning of the immune response towards a protein antigen. These results have significant implications for the next generation of rationally designed subunit vaccines.